The synaptic organizers, ?-neurexins, form macromolecular bridges with their post-synaptic partners that span the synaptic cleft (?trans-synaptic bridges?); together they play a crucial role in mediating synaptic connections and communication between neurons. ?-Neurexins and their partners are implicated in neurological disorders including autism spectrum disorder, schizophrenia and mental retardation. ?-Neurexin trans-synaptic bridges have traditionally been considered static. However, there is accumulating evidence that they are in fact dynamically regulated! Dynamic regulation of ?-neurexin trans- synaptic bridges is important, because it means that the synapse-promoting role of ?-neurexins is tunable and can be increased or decreased at a particular synapse. Three very different mechanisms have recently been revealed that control the trans-synaptic bridges between ?-neurexins and their partners: 1) competing decoys, 2) proteins secreted by astrocytes, and 3) allosteric modulation of ?-neurexin binding partners. However, it is not known on a molecular level how these mechanisms work. It is essential to determine the molecular bases that underlie these regulatory mechanisms, because they not only control the synapse-promoting activity of ?-neurexins, but they also involve protein interactions that could be targeted to manipulate specific synaptic connectivities and exploited therapeutically. In this proposal, we will determine the molecular bases of three mechanisms that control the formation of ?-neurexin trans-synaptic bridges. We hypothesize that these different molecular mechanisms exploit unique elements in the 3D structure of ?-neurexins and their partners to generate platforms that permit dynamic regulation. We will use structural, biophysical and biochemical techniques to elucidate mechanisms involving 1) competing synaptic organizers, 2) astrocytic factors, and 3) allosteric modulation. Collectively, our results will reveal on a molecular level how regulatory mechanisms mold ?-neurexin trans-synaptic bridges impacting synapse development and synaptic communication. This proposal is significant because genetic lesions in ?-neurexins and their partners are involved in the pathogenesis of severe neurological disorders, so mechanisms regulating their interactions and functions, control their contribution to disease as well. This work will also set the stage for developing mechanism-based, focused therapies to address specific CNS disorders. This proposal is conceptually innovative because it will help establish the emerging paradigm shift that trans- synaptic bridges formed by synaptic organizers, like ?-neurexins and their partners, are in fact subject to intense regulation and their ability to stabilize synaptic function is rendered tunable by other interacting proteins. This proposal is also technically innovative because it involves single particle electron tomography for which we are actively developing methodologies to image synapse organizing molecules.